The present invention relates to the driving method of switching elements using a liquid crystal for a printer and, more particularly, to the driving method of optical switching elements for a printer using a ferroelectric liquid crystal.
In recent years, investigation has been made into compact printers of the electrophotographic type, which use an optical signal generator composed of a liquid-crystal switching element and a light source. The construction of such a compact printer is shown in FIG. 1. Reference numeral 3 denotes a liquid-crystal switching element which is composed of about 2,000 shutter portions 4 of a very small area. The light emitted by a light source 6, such as a fluorescent lamp, is focused onto the surface of the liquid-crystal switching array with a cylindrical lens 5. The light transmitted through shutter portions which are selected by electrical signal, illuminates the surface of a photoconductive drum 1 through a lens system such as a linear SELFOC lens. The characteristics required for the liquid-crystal switching element includes a high-speed response. In order to obtain a line printer with a resolution of 10 dots/mm and a printing speed 1,000 lines/min. it is necessary that the on/off cycle-time is less than 1 or 2 msec.
This high-speed response cannot be obtained using twisted nematic crystals of the prior art, and a system using a twisted nematic display panel liquid-crystal element by two-frequency addressing has been proposed (in Japanese Patent Laid-Open No. 57-63509). However, this type of liquid crystal material has the problem that a cut-off frequency f.sub.c varies widely with temperature. When used as a liquid-crystal switching element for a printer, the temperature of the liquid crystals is increased (to about 40.degree. to 50.degree. C.) by the heat of the light source so that this must be compensated for. Thus, a twisted nematic liquid crystal element utilizing two frequency addressing is not suitable.
This type of liquid crystal also has another problem in that the reduction of the response time is limited to 1 to 2 msec., so that high-speed printing cannot be accomplished.